The invention relates to a method for protecting an all-wheel drive clutch of a motor vehicle with a clutch-controlled all-wheel drive and a control unit, in particular, an engine control unit or a control unit for vehicle dynamics control, with a corresponding protection function.
In a clutch-controlled all-wheel drive of a motor vehicle one axle of the vehicle is typically permanently driven, while the other axle is optionally supplied with drive power by means of a controllable all-wheel drive clutch. The driven axle is referred to as the primary axle; and the axle, which can be optionally engaged, is referred to as the secondary axle. In a clutch controlled all-wheel drive the all-wheel drive clutch is actuated preferably by electronic means. In this case a clutch torque of the all-wheel drive clutch is adjusted so that it simultaneously corresponds to the maximum possible torque that can be transmitted by the clutch in the direction of the secondary axle.
The clutch torque and, thus, also the torque of the secondary axle are limited to a maximum clutch torque of the all-wheel drive clutch, where this maximum clutch torque depends on the specific configuration of the all-wheel drive clutch. If there is a deviation between the secondary axle-sided rotational speed and the primary axle-sided rotational speed at the all-wheel drive clutch, then one speaks of clutch slip. Such a clutch slip causes a thermal energy input into the all-wheel drive clutch, so that, depending on the magnitude of the clutch slip and the duration of the thermal stress, this thermal energy input can even lead to the thermal destruction of the all-wheel drive clutch.
It is known from the German patent publication no. DE 11 2007 000 995 T5 to compare the actual clutch slip with a permissible clutch slip in a motor vehicle with all-wheel drive and then to send, as required, a signal to the engine controller, as a function of the comparison, in order to reduce the engine torque.
The U.S. Pat. No. 6,360,156 B1 describes a method for determining the differential speed between the front wheels and the rear wheels of a vehicle with an all-wheel drive and an all-wheel drive clutch. A permissible torque is determined, as a function of the differential speed; and if this permissible torque is exceeded, then the torque is reduced to a value that is not greater than the admissible torque.
A critical clutch slip may occur especially in a power braking situation, in which the primary axle exhibits a low coefficient of friction. A power braking situation means, within the context of the patent application, a situation, in which both the accelerator pedal and the brake are actuated (this is typically possible only in a vehicle with an automatic transmission) and the vehicle is stationary or moves only slightly in the longitudinal direction of the vehicle.
In a power braking situation, in which the primary axle exhibits a low coefficient of friction, the driving torque at the primary axle is typically greater than the sum of the primary axle-sided braking torque of both wheels and the wheel spin torque, so that the wheels of the primary axle spin, while the wheels of the secondary axle stand still or almost still, because the wheels of the secondary axle are locked. In a power braking situation, in which the primary axle exhibits a low coefficient of friction, the braking torque of the primary axle is often not sufficient to support (i.e., to compensate for) the high driving torque, so that the primary axle begins to spin (the effective torque on the primary axle is greater than that of the wheel spin torque of the primary axle). However, the secondary axle can fully support the torque, which can be transmitted through the all-wheel drive clutch, by the brakes of the secondary axle (since the torque, which can be transmitted to the secondary axle, is smaller), so that the wheels of the secondary axle stand still. This situation may cause the all-wheel drive clutch to slip and may also produce a high input of thermal energy without the vehicle driving off. This may result, depending on the thermal capacity of the built-in clutch, in overheating of the all-wheel drive clutch within a short period of time. To avoid a thermal destruction of the all-wheel drive system in such a case, the all-wheel drive clutch is disengaged for its protection. However, the result of such a solution is the temporary loss of the additional drive via the secondary axle.
Therefore, the object of the present invention is to provide a method, which is intended to protect an all-wheel drive clutch and which prevents a large input of energy into the all-wheel drive clutch in a critical power braking situation, when one wheel of the primary axle spins or both wheels of the primary axle spin (in particular, because the coefficient of friction of the primary axle is small), and, as a result, a clutch slip occurs. Furthermore, the object of the present invention is to provide a control unit with a corresponding protection function.
One aspect of the invention focuses on a method for protecting an all-wheel drive clutch, for example, a friction clutch (in particular, a multi-disk friction clutch), of a two-axle motor vehicle with a clutch-controlled all-wheel drive. In the all-wheel drive a primary axle is typically permanently driven; and a secondary axle can be selectively driven by means of the controllable all-wheel drive clutch. However, it is not absolutely mandatory that the primary axle be permanently driven; rather it can also be provided that the primary axle be selectively driven by means of an additional clutch.
According to the method, a power braking situation, which is critical for the all-wheel drive clutch and in which a clutch slip occurs, is detected in the all-wheel drive clutch. As explained above, both the accelerator pedal and the brake are actuated in a power braking situation. In this case the vehicle is stationary or moves only slightly in the longitudinal direction of the vehicle (for example, at a speed that is less than a threshold value, for example, less than 4 m/s). Moreover, in a critical power braking situation with clutch slip at least one of the wheels of the primary axle spins (for example, because the coefficient of friction of the primary axle is very small, for example, in the case of a primary axle on ice or wet ground) and a clutch slip occurs at the all-wheel drive clutch (because the wheels of the secondary axle are held by the brakes or barely move, while the wheels of the primary axle are spinning). Upon detection of a power braking situation, which is critical for the all-wheel drive clutch and in which a clutch slip occurs, a protective measure against a thermal overload of the all-wheel drive clutch is implemented. For example, according to an advantageous embodiment of the method, the engine torque is reduced or limited, so that the clutch slip and, thus, also the energy input into the all-wheel drive clutch are reduced. As an alternative, it may be provided that the braking torque for the at least one spinning wheel of the primary axle is increased, so that the spinning is reduced and, in so doing, the clutch slip is also reduced. It goes without saying that both the engine torque can be reduced or limited, and the braking torque can be increased.
Preferably a power braking situation, which is critical for the all-wheel drive clutch and in which a clutch slip occurs, is detected by evaluating both an accelerator pedal signal, which is characteristic for the accelerator pedal position (for example, a driving torque requested by the driver) and by evaluating a brake signal, which is characteristic for actuating the brake (for example, a braking torque requested by the driver). This approach makes it possible to check whether both the accelerator pedal and also the brake are actuated, for example, by comparing the two signals with a respective threshold value. In addition, the critical power braking situation is detected by evaluating a slip signal, which is characteristic for the clutch slip of the all-wheel drive clutch, for example, by evaluating the differential rotational speed in the all-wheel drive clutch (for example, by comparing the signal with a threshold value). Preferably a signal, which is characteristic for the vehicle speed, is also evaluated: for example, a vehicle speed signal and a rotational speed signal of a wheel of the secondary axle. For example, it is checked whether the vehicle speed in terms of its absolute value is smaller than a threshold value (for example, less than 2.5 m/s).
According to a preferred exemplary embodiment, it is also checked, in order to detect a power braking situation that is critical for the all-wheel drive clutch, whether the conditions for a critical thermal energy input into the all-wheel drive clutch are met, in particular, whether the thermal stress on the all-wheel drive clutch is increasing in a critical manner. Such a measure makes sense against the background that the clutch slip can occur in certain driving situations (for example, cornering or ABS braking), but the thermal energy input is too low and, therefore, there is no need to initiate any countermeasures. For example, it can be checked, whether the increase (i.e., the gradient) of a signal, which is characteristic for the thermal stress on the all-wheel drive clutch (for example, a temperature value), is greater than or equal to or greater than a certain threshold value.
It is advantageous to determine an engine torque, at which the wheels of the primary axle do not spin, and to reduce or limit the engine torque to this determined engine torque. As an alternative, of course, it could also be provided that the engine torque be reduced or limited to an engine torque having a smaller value than the determined engine torque. In particular, if the wheels of the primary axle are no longer spinning, then the clutch slip decreases (because the differential rotational speed between the primary axle and the secondary axle decreases); and, in so doing, the energy input into the all-wheel drive clutch decreases.
In order to determine the engine torque, at which the wheels of the primary axle do not spin, it is possible to determine, for example, the braking torque for one or both axles. As an alternative, a braking torque is estimated in terms of its absolute value downwards. For example, the braking torque can be calculated at one or both axles by means of the current brake pressure. For example, the braking torque of the secondary axle can be estimated in terms of its absolute value downwards by the clutch torque, because the all-wheel drive clutch can transmit no more torque than the clutch torque to the secondary axle. Then, when the vehicle is in stationary mode, the braking torque is greater in terms of its absolute value than the clutch torque.
It is advantageous if the method is applied to the vehicle, in which the rear axle corresponds to the primary axle, and the front axle corresponds to the secondary axle. Typically brakes with a higher maximum brake pressure are installed at the front axle rather than at the rear axle, so that when power braking, the brake pressure at the rear axle is insufficient, and the wheels of the rear axle have the tendency to spin. Thus, the method is extremely advantageous in such an all-wheel drive concept. As an alternative, the method can also naturally be applied to a vehicle, in which the rear axle corresponds to the secondary axle, and the front axle corresponds to the primary axle.
A second aspect of the invention focuses on a control unit having a function for protecting an all-wheel drive clutch of a two-axle motor vehicle with a clutch-controlled all-wheel drive. In this case the control unit may be, for example, an engine control unit or a brake control unit, in particular, a control unit for the vehicle dynamics control. The control unit comprises means for detecting a power braking situation that is critical for the all-wheel drive clutch and in which a clutch slip occurs, in the all-wheel drive clutch. Furthermore, means are provided for implementing or initiating a protective measure against a thermal overload of the all-wheel drive clutch, for example, means for reducing or limiting the engine torque and/or for increasing the braking torque for the at least one spinning wheel of the primary axle. The means are implemented, for example, in the software that is run on a processor of the control unit.
The above statements about the inventive method according to the first aspect of the invention also apply in an analogous manner to the inventive control unit according to the second aspect of the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.